1. Field of the Invention
The present invention relates to a camera system for signal-processing an output signal of an image sensor and outputting an image, and the image sensor used in the camera system.
2. Description of the Related Art
A problem of an MOS (Metal Oxide Semiconductor) image sensor was generally to generate a larger noise in comparison to a CCD (Charge Coupled Device) image sensor, however, the noise-related problem has already been improved due to an advancement in the technology. The MOS image sensor can be integrated on a single chip, downsized and easily highly integrated. Further, the MOS image sensor can be driven by a single power supply of 2.9 volts or the like, and advantageously consumes less power in comparison to the CCD image sensor. Because of the mentioned advantages, the MOS image sensor is increasingly adopted for a mobile device, a typical example of which is a camera-equipped mobile telephone, in recent years.
FIG. 8 shows a camera system in which a conventional image sensor is used. In FIG. 8, a reference numeral 100 denotes an image sensor of a line exposure comprising an imaging unit (area sensor) 101 scanned by means of an X-Y address method, a horizontal scanning unit 102 for horizontally scanning the imaging unit 101 by means of the X-Y address method, a vertical scanning unit 103 for vertically scanning the imaging unit 101 by means of the X-Y address method, and an output unit 104 for outputting an image signal So read in consequence of scanning the imaging unit 101 using the horizontal scanning unit 102 and the vertical scanning unit 103.
The imaging unit 101 has a structure in which a unit pixel is arrayed in a horizontal-vertical matrix shape. The unit pixel comprises a photoelectric conversion element for photoelectrically converting an incident light, a reading section for reading an imaging signal obtained by the photoelectric conversion element, and a reset section for resetting the photoelectric conversion element.
A reference numeral 200 denotes a signal processing circuit (DSP) comprising a CPU 201 in charge of system management, a timing generator (TG) 202 for outputting a control signal (sensor driving pulse) Sc for drive-controlling the image sensor 100 by a drive control from the CPU 201, an AE (analog front end) 203 for sampling and A/D-converting the image signal So outputted from the image sensor 100, a signal processing unit 205 for signal-processing a digital signal outputted from the AFE 203 using a memory 204 in accordance with an instruction of the CPU 201, the memory 204 for memorizing the image signal obtained by the signal processing unit 205, an I/F (interface) unit 206 for externally outputting the signal processed in the signal processing unit 205, and a memory control unit 207 for controlling the memory 204 in accordance with the instruction of the CPU 201.
The horizontal scanning unit 102 and the vertical scanning unit 103 for scanning the imaging unit 101 by means of the X-Y address method are driven by the control signal Sc of the timing generator 202.
FIGS. 9A through 9F show a motion of a photographic subject on an image photographed using the camera system of FIG. 8. In the image sensor of the line exposure such as the MOS image sensor, a light-receiving time is different in each line (focal-plane accumulation). To be more specific, the “concurrency of the accumulation” is absent in the lines in the case of the image sensor of the line exposure. As a result, when the photographic subject moves, in particular, in a same direction (parallel or non-parallel) as a main scanning direction of the imaging unit (shown by → in FIG. 9), a distortion is more possibly generated in an image. Such a phenomenon is typically generated in the image sensor of the line exposure in which a reading operation is executed in the per line.
FIGS. 9A through 9C show how the photographic subject moves in the horizontally direction to right. In FIG. 9A, a reference symbol B1 denotes the photographic subject in a Nth frame. In FIG. 9B, a reference symbol B2 denotes the photographic subject in an (N+1) th frame subsequent to the Nth frame. FIG. 9C shows an output image B3 when the motion of the photographic subject shown in FIGS. 9A and 9B is imaged by the image sensor 100 of the line exposure. As shown in FIG. 9C, such a distortion as continuous from the upper left toward the lower right is generated in the output image B3.
FIGS. 9D through 9F show how the photographic subject moves in the horizontally direction to left. In FIG. 9D, a reference symbol B4 denotes the photographic subject in the Nth frame. In FIG. 9E, a reference symbol B5 denotes the photographic subject in the (N+1) th frame. FIG. 9F shows an output image B6 when the motion of the photographic subject shown in FIGS. 9D and 9E is imaged by the image sensor 100 of the line exposure. As shown in FIG. 9F, such a distortion as continuous from the upper right toward the lower left is generated in the output image B6.
Apart from the motion of the photographic subject, it was proposed by No. 2001-358999 of the Publication of the Unexamined Japanese Patent Applications (hereinafter, referred to as conventional technology) that a motion with respect to a camera shake due to a hand movement was detected so as to calculate and change a reading position of the MOS image sensor (image sensor).
An image signal whose entire pixels are received in a same period is sequentially transferred and read in the case of the image sensor of the charge-transfer type such as the CCD. Therefore, a time difference is not generated in the light-receiving time per pixel (concurrent accumulation). In contrast to that, the line exposure for reading image information per line is generally adopted in the case of the image sensor comprising the imaging unit of the X-Y address method such as the MOS image sensor as described, wherein the light-receiving time is different in each line and the image may consequently be distorted depending on the motion of the photographic subject as described earlier.
A proposed countermeasure for dealing with the distorted image is to shorten the light-receiving time of each pixel, which, however, still cannot prevent the distortion generated between the lines.
In the conventional technology, the distortion generated by the camera shake is corrected by controlling a reading address per pixel based on the detection of the motion. However, the control operation in each line of the MOS image sensor is unfavorably complicated, and a structure of the MOS image sensor is also complicated.
Further, an example of the mobile device, such as the camera-equipped mobile telephone, has a structure in which a lens unit is rotated. In the structure, however, it is difficult to prevent the distortion generated in the image using the conventional technology alone.
An MOS image sensor of a global shutter type in which the concurrency of the accumulation is pursued is available, however, the MOS image sensor currently has too a difficult structure to be adopted for the mobile device.
Though there is a method in which the light-receiving time is adjusted by a mechanical shutter, a height of the lens unit is increased and it becomes necessary to control the mechanical shutter in adopting the method, which unfavorably increases a cost of the lens unit. Therefore, the method is not suitable for the mobile device.